1. Field of the Invention
The present invention relates to a coating method.
2. Description of Related Art
A method of coating an outer surface of a coating substrate such as a vehicle body generally includes steps: the preparatory step of removing dirt attached on the coating substrate, the coating step of spraying the coating substrate with a paint and the drying step of drying the paint coated on the coating substrate. The drying step may generally be executed at two stages: a setting step and a baking step. The setting step is usually carried out prior to the baking step at temperatures lower than those applied in the baking step, for example, in ambient atmosphere or at temperatures of 40.degree. to 60.degree. C., as is called temporary baking. The temperature in the baking step may usually be around approximately 140.degree. C.
In usual case, the coating substrate is being passed through the preparatory step, the coating step and the drying step while being transported on a transporting means such as a carriage. The coating substrate is held in a given posture at each step, in which the substrate is treated.
As one standard for evaluating the quality of a coated surface is a degree of evenness (a degree of smoothness). The greater the degree of evenness becomes, the smaller a degree of irregularities or roughness on the coated surface, thereby providing a better coated surface. In order to improve the degree of evenness, it is known that it can be done if the thickness of a coated layer, that is, the thickness of a paint coated, is made thicker.
On the contrary, the "sagging" of a paint is a factor for adversely affecting the quality of the surface of the coated substrate. The sagging arises as the coated paint flows downwardly to a large extent due to gravity, and a "sag" is more likely to occur as the thickness of a paint coated per once gets larger. The cause of the "sag" is eventually an influence of the gravity so that the sagging is likely to arise on a surface of the coating substrate extending in its vertical direction, that is, a so-called vertical surface. For example, with the body of the automotive vehicle taken into consideration as a coating substrate, a fender extending vertically is likely to cause sagging while a bonnet and a trunk lid extending transversely are unlikely to cause sagging, when coated with a paint.
Accordingly, it is possible to render the thickness of the paint thicker on a surface of the coating substrate extending in a horizontal direction, that is, a so-called horizontal surface, which does not cause problems with the "sagging" so much, than on the vertical surface. Furthermore, if the thickness of the coat layer on the horizontal surface is made equal to that on the vertical surface, irregularities on the horizontal surface is rendered smaller due to the flow of the paint coated thereon to such an extent as causing no sagging, than the vertical surface, and a higher degree of evenness is provided on the horizontal surface than on the vertical surface.
From the above point of view, heretofore, the coating is effected using a paint having the smallest possible flowability, or the lowest possible viscosity, in order to provide a coated surface with the highest possible degree of evenness while preventing the "sagging" of the coated paint. And a so-called "sagging limit" that is a limit to the thickness of the paint, which causes sagging on the vertical surface, is approximately 40 .mu.m that is the maximum that is the thickness of the coat layer for conventional thermosetting. More specifically, the "sag" of such thermosetting paints is likely to occur at the initial stages of the setting and baking steps, particularly at the initial stage of the baking step, so that the thickness of the paint to be coated in the coating step is determined so as to cause no "sag" at this stage. And the maximum thickness of the paint determined at this stage is the so-called sagging limit of 40 .mu.m. Accordingly, in order to provide a coat surface with an absolutely higher degree of evenness, conventional coating methods require, for example, a dual coating and so on, that is, a series of steps ranging from the coating step to the baking step to be repeated plural times.
U.S. Pat. Nos. 4,874,639 and 4,919,977 disclose coating methods which can provide a coat surface having a higher degree of evenness, when the film thicknesses of the two coats are the same, while overcoming the sagging limit of the paint which may cause a problem when coated by spraying in the manner as described hereinabove. More specifically, the coating method involves coating by spraying with the paint so as to form a coat layer having a film thickness thicker than its sagging limit and rotating the coated substrate about its substantially horizontal axis until the paint coated causes no sagging any longer. This coating method can provide a coat surface having a higher degree of evenness, when the thicknesses of the two coats are the same, while occurrence of the sag of the paint can be prevented, by taking positive advantage of such a high flowability of the paint used.
These coating methods, however, cause the risk that the paint sprayed may sag during the transient period from the coating step to the drying step. As long as the paint is sprayed in a film thickness thicker than its sagging limit at which it starts sagging or it is sprayed in such a film thickness by diluting the paint with a solvent or the like in these coating methods, the paint may cause the risk of sagging or dripping immediately after it has been sprayed. In this case, the sagging of the paint should be suppressed during the transient period when the coated substrate is transferred from the coating step to the drying step, if transferral of the coated substrate from the coating step to the drying step would require a certain period of time.
On the other hand, it can be noted as a matter of course that the paint can be prevented from sagging or dripping by spraying the coating substrate with the pain in a film thickness thinner than its sagging limit at which it causes sagging at least during the transient period or by diluting the paint with the solvent or the like to make flowability of the paint smaller. However, these techniques suffer from the disadvantages as conventional coating methods do and they may reduce the advantages that have been achieved with much effort by the coating methods as disclosed in the prior patents as hereinabove described, which have overcome the barrier set by a sagging limit of the paint to be sprayed.